The invention relates to a method for determining the thermal materials properties of shaped metal parts.
In certain technological processes, such as for example the cooling of hot-roll steel strips, it is necessary or desirable for the thermal materials properties of the shaped metal parts to be determined as accurately as possible. During the cooling of hot-roll steel strips, the microstructural properties of the rolled steel strips are adjusted during passage through the cooling section of the hot-rolling mill train, the microstructural properties being adjusted primarily by means of the quantity of water supplied. To calculate the quantity of water required, a cooling section model based on Fourier""s thermal conductivity equation is used. This model can be used to calculate the temperature distribution in the steel strip during the cooling operation.
One difficulty which arises in the modelling is the relatively strong dependent relationship between the thermal materials properties and the alloying elements which have been added. By way of example, the thermal conductivity at 500xc2x0 C. drops by about 50% as a result of the addition of 1% of chromium.
On account of the relatively strong dependent relationship between the thermal materials properties and the alloying elements which have been added, these elements have to be determined relatively accurately. Hitherto, this has been achieved by means of measurements at the thermodynamic equilibrium. The specific approach for the chemical dependency may, for example, involve describing the thermal conductivity of the material, which is a function of the temperature, by means of an open polygon. The coordinates of the associated supporting points are in this case made dependent on the alloying elements which are contained in the material. These dependent relationships, which are determined directly on the plant in question, are summarized in table form. In the most simple case, the sum of the alloying elements is used. Free parameters are determined by means of a compensating calculation on the basis of measurement.
Obtaining sufficiently accurate values in the method described is a very time-consuming process.
It is an object of the invention to provide a method which makes it possible, within a short time, to make it possible to sufficiently accurately determine the thermal materials properties.
This object can be achieved by a method according to the invention, in which the thermal material properties of shaped metal parts are determined from a model which describes the thermal materials properties of the shaped metal part, in which
at least one thermodynamic parameter P is formed as a linear combination of at least one basic function hi and at least one weighting factor gi in accordance with the relationship                               p          =                                    ±                                                ∑                                      i                    =                    1                                    n                                ⁢                                                      g                    i                                    ·                                      h                    i                                                                        +            c                          ,                  n          ∈          N                ,                  c          =                      const            .                          xe2x80x83                        ⁢            or                                                            p          =                      ±                                          ∑                                  i                  =                  1                                n                            ⁢                                                (                                                            g                      i                                        +                                          c                      i                                                        )                                ·                                  h                  i                                                                    ,                  n          ∈          N                ,                              c            i                    =                      const            .                          xe2x80x83                        ⁢            and                              
the basic function hi describes the thermal materials properties, and
the weighting factor gi takes account of the influence of the alloying elements on at least one thermodynamic parameter P.
The methods described in the independent claims represent a combination of a physical model and data-assisted methods for preventing a material model, the thermal materials properties being formed, according to the invention, by at least one linear combination of at least one basic function hi and at least one weighting factor gi. Therefore, the methods according to the invention do not require a concrete approach for the dependent relationship of the alloying elements, which is difficult to describe. Furthermore, the alloying elements can be taken into account in highly differentiated form. As a result, given a sufficient number of measurement data, it is possible to obtain a very accurate material model.
The thermal materials properties, which are defined by the thermodynamic parameter p and can be described by at least one basic function, are, for example, the enthalpy, the heat capacity and the thermal conductivity.
In the context of the invention, the basic functions used may be, for example, as alternatives or in combination, rectangular blocks, sawteeth, B splines, preferably 4th order B splines, or Gaussian bells.
If the linear combination consists exclusively of rectangular blocks, a step function results for the thermodynamic parameter p.
Given a linear combination exclusively comprising sawteeth (first order B splines), an open polygon is obtained for the thermodynamic parameter p.
If exclusively 4th order B splines are used to form a linear combination, a cubic spline is obtained for the thermodynamic parameter p.
In an advantageous configuration of the method according to the invention, the weighting factors gi are determined in a neural network from the mass contents of the alloying elements which have been added and/or variables derived therefrom.
A predeterminable number of data sets, which each include measurements of surface temperatures and velocity and information about the quantities of water required for cooling for a specific hot-rolled steel strip, is used to train the neural network. The respective steel strips mayxe2x80x94but do not have toxe2x80x94in this case have different concentrations of alloying elements.
The data sets include measurements for at least approximately pure iron. A model is determined from these parts of the data sets and is matched to reality. The thermodynamic parameters for pure iron are known, and consequently matching of the model to reality is carried out only for the heat transfer. Then, the entire data set, which also includes measurements for alloyed steels, is taken into consideration. A possible deviation from the model which has previously been matched to reality is determined from this data set. This deviation can only be explained by alloying contents which are present in the material, and not by the cooling operation (water quantity and water temperature). The calculations are matched to the measurements by changing the network weights wi. The change in the network weights Wi leads to a change in the weighting factors gi. The calculations are matched to the measurements until the calculations correspond sufficiently well to the measurements for all the shaped parts (e.g. steel strips).
As an alternative to determining weighting factors gi in a neural network, it is possible for weighting factors gi to be determined by a linear combination of the mass content of at least one alloying element and/or variables derived therefrom and a regression factor.